[PATCH v2 1/3] doc: Move devicetree control doc to rST

[PATCH v2 1/3] doc: Move devicetree control doc to rST

[Simon Glass]

Move this to rST format, largely unchanged to start with. Add an index
for this topic, as well as an empty intro.

Note this patch does not include updates! Is it just a conversion to the
new format. See the next patch.

Signed-off-by: Simon Glass <sjg@chromium.org>
---

Changes in v2:
- Add a comment about updates

 .../devicetree/control.rst}                   | 50 +++++++++----------
 doc/develop/devicetree/index.rst              | 13 +++++
 doc/develop/devicetree/intro.rst              |  4 ++
 doc/develop/index.rst                         |  1 +
 4 files changed, 43 insertions(+), 25 deletions(-)
 rename doc/{README.fdt-control => develop/devicetree/control.rst} (89%)
 create mode 100644 doc/develop/devicetree/index.rst
 create mode 100644 doc/develop/devicetree/intro.rst

diff --git a/doc/README.fdt-control b/doc/develop/devicetree/control.rst
similarity index 89%
rename from doc/README.fdt-control
rename to doc/develop/devicetree/control.rst
index 424d13fc5b1..1289b6156fe 100644
--- a/doc/README.fdt-control
+++ b/doc/develop/devicetree/control.rst
@@ -1,6 +1,5 @@
-# SPDX-License-Identifier: GPL-2.0+
-#
-# Copyright (c) 2011 The Chromium OS Authors.
+.. SPDX-License-Identifier: GPL-2.0+
+.. sectionauthor:: Copyright 2011 The Chromium OS Authors
 
 Device Tree Control in U-Boot
 =============================
@@ -37,9 +36,7 @@ What is a Flat Device Tree?
 ---------------------------
 
 An fdt can be specified in source format as a text file. To read about
-the fdt syntax, take a look at the specification here:
-
-https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
+the fdt syntax, take a look at the specification (dtspec_).
 
 You also might find this section of the Linux kernel documentation
 useful: (access this in the Linux kernel source code)
@@ -60,18 +57,18 @@ To use this feature you will need to get the device tree compiler. This is
 provided by U-Boot automatically. If you have a system version of dtc
 (typically in the 'device-tree-compiler' package), it is currently not used.
 
-If you want to build your own dtc, it is kept here:
+If you want to build your own dtc, it is kept here::
 
 	git://git.kernel.org/pub/scm/utils/dtc/dtc.git
 
-For example:
+For example::
 
 	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
 	$ cd dtc
 	$ make
 	$ sudo make install
 
-Then run the compiler (your version will vary):
+Then run the compiler (your version will vary)::
 
 	$ dtc -v
 	Version: DTC 1.2.0-g2cb4b51f
@@ -105,14 +102,14 @@ Failing that, you could write one from scratch yourself!
 Configuration
 -------------
 
-Use:
+Use::
 
-#define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
+   #define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
 
 to set the filename of the device tree source. Then put your device tree
-file into
+file into::
 
-	board/<vendor>/dts/<name>.dts
+   board/<vendor>/dts/<name>.dts
 
 This should include your CPU or SOC's device tree file, placed in
 arch/<arch>/dts, and then make any adjustments required.
@@ -123,9 +120,9 @@ and development only and is not recommended for production devices.
 
 If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
 a u-boot.dtb file alongside u-boot-nodtb.bin. A common approach is then to
-join the two:
+join the two::
 
-	cat u-boot-nodtb.bin u-boot.dtb >image.bin
+   cat u-boot-nodtb.bin u-boot.dtb >image.bin
 
 and then flash image.bin onto your board. Note that U-Boot creates
 u-boot-dtb.bin which does the above step for you also. Resulting
@@ -144,9 +141,9 @@ specify the file to read.
 You cannot use more than one of these options at the same time.
 
 To use a device tree file that you have compiled yourself, pass
-EXT_DTB=<filename> to 'make', as in:
+EXT_DTB=<filename> to 'make', as in::
 
-	make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
+   make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
 
 Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
 if used, and u-boot-dtb.bin.
@@ -162,16 +159,21 @@ variable will be set to the address of the newly relocated fdt blob.
 It is read-only and cannot be changed. It can optionally be used to
 control the boot process of Linux with bootm/bootz commands.
 
-To use this, put something like this in your board header file:
+To use this, put something like this in your board header file::
 
-#define CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"
+   #define CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"
 
 Build:
 
-After board configuration is done, fdt supported u-boot can be build in two ways:
-1)  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
+After board configuration is done, fdt supported u-boot can be build in two
+ways:
+
+#  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE::
+
     $ make
-2)  build the user specified dts file
+
+#  build the user specified dts file::
+
     $ make DEVICE_TREE=<dts-file-name>
 
 
@@ -225,6 +227,4 @@ but can use the fdt to specific the UART clock, peripheral address, etc.
 In very broad terms, the CONFIG options in general control *what* driver
 files are pulled in, and the fdt controls *how* those files work.
 
---
-Simon Glass <sjg@chromium.org>
-1-Sep-11
+.. _dtspec: https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
diff --git a/doc/develop/devicetree/index.rst b/doc/develop/devicetree/index.rst
new file mode 100644
index 00000000000..fa5db3eb76e
--- /dev/null
+++ b/doc/develop/devicetree/index.rst
@@ -0,0 +1,13 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Devicetree in U-Boot
+====================
+
+The following holds information on how U-Boot makes use of devicetree for
+build-time and runtime configuration.
+
+.. toctree::
+   :maxdepth: 2
+
+   intro
+   control
diff --git a/doc/develop/devicetree/intro.rst b/doc/develop/devicetree/intro.rst
new file mode 100644
index 00000000000..344851327c7
--- /dev/null
+++ b/doc/develop/devicetree/intro.rst
@@ -0,0 +1,4 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Devicetree Introduction
+=======================
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 3ead7bda8fd..3230ff9cf0e 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -10,6 +10,7 @@ Implementation
    :maxdepth: 1
 
    commands
+   devicetree/index
    driver-model/index
    global_data
    logging
-- 
2.32.0.554.ge1b32706d8-goog

[PATCH v2 2/3] doc: Update devicedocs including how to add tweaks

[Simon Glass]

This file is about 10 years old and the updates have not covered
everything that has changed, particularly in the last few years. Update
the information and add mention of the u-boot.dtsi files.

Signed-off-by: Simon Glass <sjg@chromium.org>
---

Changes in v2:
- Update many, many things as suggested by Heinrich

 doc/develop/devicetree/control.rst | 181 ++++++++++++++++-------------
 1 file changed, 100 insertions(+), 81 deletions(-)

diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst
index 1289b6156fe..9f2c21034d0 100644
--- a/doc/develop/devicetree/control.rst
+++ b/doc/develop/devicetree/control.rst
@@ -1,93 +1,72 @@
 .. SPDX-License-Identifier: GPL-2.0+
 .. sectionauthor:: Copyright 2011 The Chromium OS Authors
 
-Device Tree Control in U-Boot
-=============================
+Devicetree Control in U-Boot
+============================
 
-This feature provides for run-time configuration of U-Boot via a flat
-device tree (fdt). U-Boot configuration has traditionally been done
-using CONFIG options in the board config file. This feature aims to
-make it possible for a single U-Boot binary to support multiple boards,
-with the exact configuration of each board controlled by a flat device
-tree (fdt). This is the approach recently taken by the ARM Linux kernel
-and has been used by PowerPC for some time.
+This feature provides for run-time configuration of U-Boot via a flattened
+devicetree (fdt).
+
+This feature aims to make it possible for a single U-Boot binary to support
+multiple boards, with the exact configuration of each board controlled by
+a flattened devicetree (fdt). This is the approach  taken by Linux kernel for
+ARM and Risc-V and has been used by PowerPC for some time.
 
 The fdt is a convenient vehicle for implementing run-time configuration
-for three reasons. Firstly it is easy to use, being a simple text file.
-It is extensible since it consists of nodes and properties in a nice
-hierarchical format.
+for three reasons:
 
-Finally, there is already excellent infrastructure for the fdt: a
-compiler checks the text file and converts it to a compact binary
-format, and a library is already available in U-Boot (libfdt) for
-handling this format.
+- There is already excellent infrastructure for the fdt: a compiler checks
+  the text file and converts it to a compact binary format, and a library
+  is already available in U-Boot (libfdt) for handling this format
+- It is extensible since it consists of nodes and properties in a nice
+  hierarchical format
+- It is fairly efficient to read incrementally
 
-The dts directory contains a Makefile for building the device tree blob
-and embedding it in your U-Boot image. This is useful since it allows
+The arch/<arch>/dts directories contains a Makefile for building the devicetree
+blob and embedding it in the U-Boot image. This is useful since it allows
 U-Boot to configure itself according to what it finds there. If you have
 a number of similar boards with different peripherals, you can describe
-the features of each board in the device tree file, and have a single
+the features of each board in the devicetree file, and have a single
 generic source base.
 
 To enable this feature, add CONFIG_OF_CONTROL to your board config file.
 
 
-What is a Flat Device Tree?
----------------------------
+What is a Flattened Devicetree?
+-------------------------------
 
 An fdt can be specified in source format as a text file. To read about
 the fdt syntax, take a look at the specification (dtspec_).
 
-You also might find this section of the Linux kernel documentation
-useful: (access this in the Linux kernel source code)
-
-	Documentation/devicetree/booting-without-of.txt
-
-There is also a mailing list:
+There is also a mailing list (dtlist_) for the compiler and associated
+tools.
 
-	http://lists.ozlabs.org/listinfo/devicetree-discuss
-
-In case you are wondering, OF stands for Open Firmware.
+In case you are wondering, OF stands for Open Firmware. This follows the
+convention used in Linux.
 
 
 Tools
 -----
 
-To use this feature you will need to get the device tree compiler. This is
+To create flattened device trees the device tree compiler is used. This is
 provided by U-Boot automatically. If you have a system version of dtc
-(typically in the 'device-tree-compiler' package), it is currently not used.
+(typically in the 'device-tree-compiler' package), that system version is
+currently not used.
 
 If you want to build your own dtc, it is kept here::
 
-	git://git.kernel.org/pub/scm/utils/dtc/dtc.git
-
-For example::
+    git://git.kernel.org/pub/scm/utils/dtc/dtc.git
 
-	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
-	$ cd dtc
-	$ make
-	$ sudo make install
+You can decode a binary file with::
 
-Then run the compiler (your version will vary)::
+    dtc -I dtb -O dts <filename.dtb>
 
-	$ dtc -v
-	Version: DTC 1.2.0-g2cb4b51f
-	$ make tests
-	$ cd tests
-	$ ./run_tests.sh
-	********** TEST SUMMARY
-	*     Total testcases:	1371
-	*                PASS:	1371
-	*                FAIL:	0
-	*   Bad configuration:	0
-	* Strange test result:	0
+That repo also includes `fdtget`/`fdtput` for reading and writing properties in
+a binary file. U-Boot adds its own `fdtgrep` for creating subsets of the file.
 
-You will also find a useful fdtdump utility for decoding a binary file, as
-well as fdtget/fdtput for reading and writing properties in a binary file.
 
-
-Where do I get an fdt file for my board?
-----------------------------------------
+Where do I get an devicetree file for my board?
+-----------------------------------------------
 
 You may find that the Linux kernel has a suitable file. Look in the
 kernel source in arch/<arch>/boot/dts.
@@ -106,41 +85,37 @@ Use::
 
    #define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
 
-to set the filename of the device tree source. Then put your device tree
+to set the filename of the devicetree source. Then put your devicetree
 file into::
 
-   board/<vendor>/dts/<name>.dts
+   arch/<arch>/dts/<name>.dts
 
-This should include your CPU or SOC's device tree file, placed in
-arch/<arch>/dts, and then make any adjustments required.
+This should include your CPU or SOC's devicetree file, placed in
+`arch/<arch>/dts`, and then make any adjustments required using a u-boot-dtsi
+file for your board.
 
 If CONFIG_OF_EMBED is defined, then it will be picked up and built into
 the U-Boot image (including u-boot.bin). This is suitable for debugging
 and development only and is not recommended for production devices.
 
 If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
-a u-boot.dtb file alongside u-boot-nodtb.bin. A common approach is then to
-join the two::
-
-   cat u-boot-nodtb.bin u-boot.dtb >image.bin
-
-and then flash image.bin onto your board. Note that U-Boot creates
-u-boot-dtb.bin which does the above step for you also. Resulting
-u-boot.bin is a copy of u-boot-dtb.bin in this case. If you are using
-CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
+a u-boot.dtb file alongside u-boot-nodtb.bin with the combined result placed
+in u-boot.bin so you can still just flash u-boot,bin onto your board. If you are
+using CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
 tree binary.
 
 If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
-device tree at runtime, for example if an earlier bootloader stage creates
+devicetree at runtime, for example if an earlier bootloader stage creates
 it and passes it to U-Boot.
 
 If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
 startup. This is only useful for sandbox. Use the -d flag to U-Boot to
-specify the file to read.
+specify the file to read, -D for the default and -T for the test devicetree,
+used to run sandbox unit tests.
 
 You cannot use more than one of these options at the same time.
 
-To use a device tree file that you have compiled yourself, pass
+To use a devicetree file that you have compiled yourself, pass
 EXT_DTB=<filename> to 'make', as in::
 
    make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
@@ -165,7 +140,7 @@ To use this, put something like this in your board header file::
 
 Build:
 
-After board configuration is done, fdt supported u-boot can be build in two
+After the board configuration is done, fdt supported u-boot can be built in two
 ways:
 
 #  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE::
@@ -177,12 +152,41 @@ ways:
     $ make DEVICE_TREE=<dts-file-name>
 
 
+Adding tweaks for U-Boot
+------------------------
+
+It is strongly recommended that devicetree files in U-Boot be an exact copy of
+those in Linux, so that it is easy to sync them up from time to time.
+
+U-Boot is of course a very different project from Linux, e.g. it operates under
+much more restrictive memory and code-size constraints. Where Linux may use a
+full clock driver with Common Clock Format (CCF) to find the input clock to the
+UART, U-Boot typically wants to output a banner as early as possible before too
+much code has run.
+
+A second difference is that U-Boot includes different phases. For SPL,
+constraints are even more extreme and the devicetree is shrunk to remove
+unwanted nodes, or even turned into C code to avoid access overhead.
+
+U-Boot automatically looks for and incluces a file with updates to the standard
+devicetree for your board, searching for them in the same directory as the
+main file, in this order::
+
+   <orig_filename>-u-boot.dtsi
+   <CONFIG_SYS_SOC>-u-boot.dtsi
+   <CONFIG_SYS_CPU>-u-boot.dtsi
+   <CONFIG_SYS_VENDOR>-u-boot.dtsi
+   u-boot.dtsi
+
+Only one of these is selected but of course you can #include another one within
+that file, to create a hierarchy of shared files.
+
 Relocation, SPL and TPL
 -----------------------
 
 U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.
 
-The full device tree is available to U-Boot proper, but normally only a subset
+The full devicetree is available to U-Boot proper, but normally only a subset
 (or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and
 'SPL Support' in doc/driver-model/design.rst for more details.
 
@@ -201,24 +205,24 @@ If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM
 containing the board ID for example), it possible to start with a generic DTB
 and then switch over to the right DTB after the detection. For this purpose,
 the platform code must call fdtdec_resetup(). Based on the returned flag, the
-platform may have to re-initiliaze the DM subusystem using dm_uninit() and
+platform may have to re-initialise the DM subsystem using dm_uninit() and
 dm_init_and_scan().
 
 
 Limitations
 -----------
 
-U-Boot is designed to build with a single architecture type and CPU
+Devicetree can help reduce the complexity of supporting variants of boards which
+use the same SOC / CPU.
+
+However U-Boot is designed to build for a single architecture type and CPU
 type. So for example it is not possible to build a single ARM binary
 which runs on your AT91 and OMAP boards, relying on an fdt to configure
 the various features. This is because you must select one of
 the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
-time. Similarly you cannot build for multiple cpu types or
+time. Similarly U-Boot cannot be built for multiple cpu types or
 architectures.
 
-That said the complexity reduction by using fdt to support variants of
-boards which use the same SOC / CPU can be substantial.
-
 It is important to understand that the fdt only selects options
 available in the platform / drivers. It cannot add new drivers (yet). So
 you must still have the CONFIG option to enable the driver. For example,
@@ -227,4 +231,19 @@ but can use the fdt to specific the UART clock, peripheral address, etc.
 In very broad terms, the CONFIG options in general control *what* driver
 files are pulled in, and the fdt controls *how* those files work.
 
-.. _dtspec: https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
+History
+-------
+
+U-Boot configuration was previous done using CONFIG options in the board
+config file. This eventually got out of hand with nearly 10,000 options.
+
+U-Boot adopted devicetree around the same time as Linux and early boards
+used it before Linux (e.g. snow). The two projects developed in parallel
+and there are still some differences in the bindings with certain boards.
+While there has beeen discussion of having a separate repo for devicetree
+files, in practice the Linux kernel Git repository has become the place where
+these are stored, with U-Boot taking copies and adding tweaks with u-boot.dtsi
+files.
+
+.. _dtspec: https://www.devicetree.org/specifications/
+.. _dtlist: https://www.spinics.net/lists/devicetree-compiler/
-- 
2.32.0.554.ge1b32706d8-goog

[PATCH v2 3/3] doc: Add a note about why devicetree is used

[Simon Glass]

This question comes up every now and then with people coming from Linux.
Add some notes about it so we can point to it in the mailing list.

Signed-off-by: Simon Glass <sjg@chromium.org>
---

Changes in v2:
- Updates based on suggestions from Heinrich

 doc/develop/devicetree/control.rst |  2 ++
 doc/develop/devicetree/intro.rst   | 40 ++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst
index 9f2c21034d0..b33e2a7ead6 100644
--- a/doc/develop/devicetree/control.rst
+++ b/doc/develop/devicetree/control.rst
@@ -152,6 +152,8 @@ ways:
     $ make DEVICE_TREE=<dts-file-name>
 
 
+.. _dttweaks:
+
 Adding tweaks for U-Boot
 ------------------------
 
diff --git a/doc/develop/devicetree/intro.rst b/doc/develop/devicetree/intro.rst
index 344851327c7..36e8cc0d440 100644
--- a/doc/develop/devicetree/intro.rst
+++ b/doc/develop/devicetree/intro.rst
@@ -2,3 +2,43 @@
 
 Devicetree Introduction
 =======================
+
+U-Boot uses a devicetree for configuration. This includes the devices used by
+the board, the format of the image created with binman, which UART to use for
+the console, public keys used for secure boot and many other things.
+
+See :doc:`control` for more information.
+
+Why does U-Boot put <thing> in the devicetree?
+----------------------------------------------
+
+This question comes up a lot with people new to U-Boot, particular those coming
+from Linux who are used to quite strict rules about what can go into the
+devicetree.
+
+U-Boot uses the same devicetree as Linux but adds more things necessary for the
+bootloader environment (see :ref:`dttweaks`).
+
+U-Boot does not have a user space to provide policy and configuration. It cannot
+do what Linux does and run programs and look up filesystems to figure out how to
+boot. So configuration and runtime information goes into the devicetree in
+U-Boot.
+
+Of course it is possible to:
+
+- add tables into the rodata section of the U-Boot binary
+- append some info to the end of U-Boot in a different format
+- modify the linker script to bring in a file with some info in it
+- put things in ACPI tables
+- link in a UEFI hand-off block structure and put things in there
+
+but *please don't*. In general, devicetree is the sane place to hold U-Boot's
+configuration.
+
+So, please, do NOT ask why U-Boot puts <thing> in the devicetree. It is the only
+place it can go. It is a highly suitable data structure for just about anything
+that U-Boot needs to know at runtime.
+
+Note, it is possible to use platdata directly so drivers avoid devicetreee in
+SPL. But of-platdata is the modern way of avoiding devicetree overhead, so
+please use that instead.
-- 
2.32.0.554.ge1b32706d8-goog

Re: [PATCH v2 1/3] doc: Move devicetree control doc to rST

[Heinrich Schuchardt]

On 8/2/21 2:57 AM, Simon Glass wrote:
> Move this to rST format, largely unchanged to start with. Add an index
> for this topic, as well as an empty intro.
>
> Note this patch does not include updates! Is it just a conversion to the
> new format. See the next patch.
>
> Signed-off-by: Simon Glass <sjg@chromium.org>

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

> ---
>
> Changes in v2:
> - Add a comment about updates
>
>   .../devicetree/control.rst}                   | 50 +++++++++----------
>   doc/develop/devicetree/index.rst              | 13 +++++
>   doc/develop/devicetree/intro.rst              |  4 ++
>   doc/develop/index.rst                         |  1 +
>   4 files changed, 43 insertions(+), 25 deletions(-)
>   rename doc/{README.fdt-control => develop/devicetree/control.rst} (89%)
>   create mode 100644 doc/develop/devicetree/index.rst
>   create mode 100644 doc/develop/devicetree/intro.rst
>
> diff --git a/doc/README.fdt-control b/doc/develop/devicetree/control.rst
> similarity index 89%
> rename from doc/README.fdt-control
> rename to doc/develop/devicetree/control.rst
> index 424d13fc5b1..1289b6156fe 100644
> --- a/doc/README.fdt-control
> +++ b/doc/develop/devicetree/control.rst
> @@ -1,6 +1,5 @@
> -# SPDX-License-Identifier: GPL-2.0+
> -#
> -# Copyright (c) 2011 The Chromium OS Authors.
> +.. SPDX-License-Identifier: GPL-2.0+
> +.. sectionauthor:: Copyright 2011 The Chromium OS Authors
>
>   Device Tree Control in U-Boot
>   =============================
> @@ -37,9 +36,7 @@ What is a Flat Device Tree?
>   ---------------------------
>
>   An fdt can be specified in source format as a text file. To read about
> -the fdt syntax, take a look at the specification here:
> -
> -https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
> +the fdt syntax, take a look at the specification (dtspec_).
>
>   You also might find this section of the Linux kernel documentation
>   useful: (access this in the Linux kernel source code)
> @@ -60,18 +57,18 @@ To use this feature you will need to get the device tree compiler. This is
>   provided by U-Boot automatically. If you have a system version of dtc
>   (typically in the 'device-tree-compiler' package), it is currently not used.
>
> -If you want to build your own dtc, it is kept here:
> +If you want to build your own dtc, it is kept here::
>
>   	git://git.kernel.org/pub/scm/utils/dtc/dtc.git
>
> -For example:
> +For example::
>
>   	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
>   	$ cd dtc
>   	$ make
>   	$ sudo make install
>
> -Then run the compiler (your version will vary):
> +Then run the compiler (your version will vary)::
>
>   	$ dtc -v
>   	Version: DTC 1.2.0-g2cb4b51f
> @@ -105,14 +102,14 @@ Failing that, you could write one from scratch yourself!
>   Configuration
>   -------------
>
> -Use:
> +Use::
>
> -#define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
> +   #define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
>
>   to set the filename of the device tree source. Then put your device tree
> -file into
> +file into::
>
> -	board/<vendor>/dts/<name>.dts
> +   board/<vendor>/dts/<name>.dts
>
>   This should include your CPU or SOC's device tree file, placed in
>   arch/<arch>/dts, and then make any adjustments required.
> @@ -123,9 +120,9 @@ and development only and is not recommended for production devices.
>
>   If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
>   a u-boot.dtb file alongside u-boot-nodtb.bin. A common approach is then to
> -join the two:
> +join the two::
>
> -	cat u-boot-nodtb.bin u-boot.dtb >image.bin
> +   cat u-boot-nodtb.bin u-boot.dtb >image.bin
>
>   and then flash image.bin onto your board. Note that U-Boot creates
>   u-boot-dtb.bin which does the above step for you also. Resulting
> @@ -144,9 +141,9 @@ specify the file to read.
>   You cannot use more than one of these options at the same time.
>
>   To use a device tree file that you have compiled yourself, pass
> -EXT_DTB=<filename> to 'make', as in:
> +EXT_DTB=<filename> to 'make', as in::
>
> -	make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
> +   make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
>
>   Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
>   if used, and u-boot-dtb.bin.
> @@ -162,16 +159,21 @@ variable will be set to the address of the newly relocated fdt blob.
>   It is read-only and cannot be changed. It can optionally be used to
>   control the boot process of Linux with bootm/bootz commands.
>
> -To use this, put something like this in your board header file:
> +To use this, put something like this in your board header file::
>
> -#define CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"
> +   #define CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"
>
>   Build:
>
> -After board configuration is done, fdt supported u-boot can be build in two ways:
> -1)  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
> +After board configuration is done, fdt supported u-boot can be build in two
> +ways:
> +
> +#  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE::
> +
>       $ make
> -2)  build the user specified dts file
> +
> +#  build the user specified dts file::
> +
>       $ make DEVICE_TREE=<dts-file-name>
>
>
> @@ -225,6 +227,4 @@ but can use the fdt to specific the UART clock, peripheral address, etc.
>   In very broad terms, the CONFIG options in general control *what* driver
>   files are pulled in, and the fdt controls *how* those files work.
>
> ---
> -Simon Glass <sjg@chromium.org>
> -1-Sep-11
> +.. _dtspec: https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
> diff --git a/doc/develop/devicetree/index.rst b/doc/develop/devicetree/index.rst
> new file mode 100644
> index 00000000000..fa5db3eb76e
> --- /dev/null
> +++ b/doc/develop/devicetree/index.rst
> @@ -0,0 +1,13 @@
> +.. SPDX-License-Identifier: GPL-2.0+
> +
> +Devicetree in U-Boot
> +====================
> +
> +The following holds information on how U-Boot makes use of devicetree for
> +build-time and runtime configuration.
> +
> +.. toctree::
> +   :maxdepth: 2
> +
> +   intro
> +   control
> diff --git a/doc/develop/devicetree/intro.rst b/doc/develop/devicetree/intro.rst
> new file mode 100644
> index 00000000000..344851327c7
> --- /dev/null
> +++ b/doc/develop/devicetree/intro.rst
> @@ -0,0 +1,4 @@
> +.. SPDX-License-Identifier: GPL-2.0+
> +
> +Devicetree Introduction
> +=======================
> diff --git a/doc/develop/index.rst b/doc/develop/index.rst
> index 3ead7bda8fd..3230ff9cf0e 100644
> --- a/doc/develop/index.rst
> +++ b/doc/develop/index.rst
> @@ -10,6 +10,7 @@ Implementation
>      :maxdepth: 1
>
>      commands
> +   devicetree/index
>      driver-model/index
>      global_data
>      logging
>

Re: [PATCH v2 2/3] doc: Update devicedocs including how to add tweaks

[Heinrich Schuchardt]

On 8/2/21 2:57 AM, Simon Glass wrote:
> This file is about 10 years old and the updates have not covered
> everything that has changed, particularly in the last few years. Update
> the information and add mention of the u-boot.dtsi files.
>
> Signed-off-by: Simon Glass <sjg@chromium.org>

I will fix a few typos when merging, e.g.

Risc-V -> RISC-V
beeen -> been
repo -> repository

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

> ---
>
> Changes in v2:
> - Update many, many things as suggested by Heinrich
>
>   doc/develop/devicetree/control.rst | 181 ++++++++++++++++-------------
>   1 file changed, 100 insertions(+), 81 deletions(-)
>
> diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst
> index 1289b6156fe..9f2c21034d0 100644
> --- a/doc/develop/devicetree/control.rst
> +++ b/doc/develop/devicetree/control.rst
> @@ -1,93 +1,72 @@
>   .. SPDX-License-Identifier: GPL-2.0+
>   .. sectionauthor:: Copyright 2011 The Chromium OS Authors
>
> -Device Tree Control in U-Boot
> -=============================
> +Devicetree Control in U-Boot
> +============================
>
> -This feature provides for run-time configuration of U-Boot via a flat
> -device tree (fdt). U-Boot configuration has traditionally been done
> -using CONFIG options in the board config file. This feature aims to
> -make it possible for a single U-Boot binary to support multiple boards,
> -with the exact configuration of each board controlled by a flat device
> -tree (fdt). This is the approach recently taken by the ARM Linux kernel
> -and has been used by PowerPC for some time.
> +This feature provides for run-time configuration of U-Boot via a flattened
> +devicetree (fdt).
> +
> +This feature aims to make it possible for a single U-Boot binary to support
> +multiple boards, with the exact configuration of each board controlled by
> +a flattened devicetree (fdt). This is the approach  taken by Linux kernel for
> +ARM and Risc-V and has been used by PowerPC for some time.
>
>   The fdt is a convenient vehicle for implementing run-time configuration
> -for three reasons. Firstly it is easy to use, being a simple text file.
> -It is extensible since it consists of nodes and properties in a nice
> -hierarchical format.
> +for three reasons:
>
> -Finally, there is already excellent infrastructure for the fdt: a
> -compiler checks the text file and converts it to a compact binary
> -format, and a library is already available in U-Boot (libfdt) for
> -handling this format.
> +- There is already excellent infrastructure for the fdt: a compiler checks
> +  the text file and converts it to a compact binary format, and a library
> +  is already available in U-Boot (libfdt) for handling this format
> +- It is extensible since it consists of nodes and properties in a nice
> +  hierarchical format
> +- It is fairly efficient to read incrementally
>
> -The dts directory contains a Makefile for building the device tree blob
> -and embedding it in your U-Boot image. This is useful since it allows
> +The arch/<arch>/dts directories contains a Makefile for building the devicetree
> +blob and embedding it in the U-Boot image. This is useful since it allows
>   U-Boot to configure itself according to what it finds there. If you have
>   a number of similar boards with different peripherals, you can describe
> -the features of each board in the device tree file, and have a single
> +the features of each board in the devicetree file, and have a single
>   generic source base.
>
>   To enable this feature, add CONFIG_OF_CONTROL to your board config file.
>
>
> -What is a Flat Device Tree?
> ----------------------------
> +What is a Flattened Devicetree?
> +-------------------------------
>
>   An fdt can be specified in source format as a text file. To read about
>   the fdt syntax, take a look at the specification (dtspec_).
>
> -You also might find this section of the Linux kernel documentation
> -useful: (access this in the Linux kernel source code)
> -
> -	Documentation/devicetree/booting-without-of.txt
> -
> -There is also a mailing list:
> +There is also a mailing list (dtlist_) for the compiler and associated
> +tools.
>
> -	http://lists.ozlabs.org/listinfo/devicetree-discuss
> -
> -In case you are wondering, OF stands for Open Firmware.
> +In case you are wondering, OF stands for Open Firmware. This follows the
> +convention used in Linux.
>
>
>   Tools
>   -----
>
> -To use this feature you will need to get the device tree compiler. This is
> +To create flattened device trees the device tree compiler is used. This is
>   provided by U-Boot automatically. If you have a system version of dtc
> -(typically in the 'device-tree-compiler' package), it is currently not used.
> +(typically in the 'device-tree-compiler' package), that system version is
> +currently not used.
>
>   If you want to build your own dtc, it is kept here::
>
> -	git://git.kernel.org/pub/scm/utils/dtc/dtc.git
> -
> -For example::
> +    git://git.kernel.org/pub/scm/utils/dtc/dtc.git
>
> -	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
> -	$ cd dtc
> -	$ make
> -	$ sudo make install
> +You can decode a binary file with::
>
> -Then run the compiler (your version will vary)::
> +    dtc -I dtb -O dts <filename.dtb>
>
> -	$ dtc -v
> -	Version: DTC 1.2.0-g2cb4b51f
> -	$ make tests
> -	$ cd tests
> -	$ ./run_tests.sh
> -	********** TEST SUMMARY
> -	*     Total testcases:	1371
> -	*                PASS:	1371
> -	*                FAIL:	0
> -	*   Bad configuration:	0
> -	* Strange test result:	0
> +That repo also includes `fdtget`/`fdtput` for reading and writing properties in
> +a binary file. U-Boot adds its own `fdtgrep` for creating subsets of the file.
>
> -You will also find a useful fdtdump utility for decoding a binary file, as
> -well as fdtget/fdtput for reading and writing properties in a binary file.
>
> -
> -Where do I get an fdt file for my board?
> -----------------------------------------
> +Where do I get an devicetree file for my board?
> +-----------------------------------------------
>
>   You may find that the Linux kernel has a suitable file. Look in the
>   kernel source in arch/<arch>/boot/dts.
> @@ -106,41 +85,37 @@ Use::
>
>      #define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
>
> -to set the filename of the device tree source. Then put your device tree
> +to set the filename of the devicetree source. Then put your devicetree
>   file into::
>
> -   board/<vendor>/dts/<name>.dts
> +   arch/<arch>/dts/<name>.dts
>
> -This should include your CPU or SOC's device tree file, placed in
> -arch/<arch>/dts, and then make any adjustments required.
> +This should include your CPU or SOC's devicetree file, placed in
> +`arch/<arch>/dts`, and then make any adjustments required using a u-boot-dtsi
> +file for your board.
>
>   If CONFIG_OF_EMBED is defined, then it will be picked up and built into
>   the U-Boot image (including u-boot.bin). This is suitable for debugging
>   and development only and is not recommended for production devices.
>
>   If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
> -a u-boot.dtb file alongside u-boot-nodtb.bin. A common approach is then to
> -join the two::
> -
> -   cat u-boot-nodtb.bin u-boot.dtb >image.bin
> -
> -and then flash image.bin onto your board. Note that U-Boot creates
> -u-boot-dtb.bin which does the above step for you also. Resulting
> -u-boot.bin is a copy of u-boot-dtb.bin in this case. If you are using
> -CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
> +a u-boot.dtb file alongside u-boot-nodtb.bin with the combined result placed
> +in u-boot.bin so you can still just flash u-boot,bin onto your board. If you are
> +using CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
>   tree binary.
>
>   If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
> -device tree at runtime, for example if an earlier bootloader stage creates
> +devicetree at runtime, for example if an earlier bootloader stage creates
>   it and passes it to U-Boot.
>
>   If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
>   startup. This is only useful for sandbox. Use the -d flag to U-Boot to
> -specify the file to read.
> +specify the file to read, -D for the default and -T for the test devicetree,
> +used to run sandbox unit tests.
>
>   You cannot use more than one of these options at the same time.
>
> -To use a device tree file that you have compiled yourself, pass
> +To use a devicetree file that you have compiled yourself, pass
>   EXT_DTB=<filename> to 'make', as in::
>
>      make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
> @@ -165,7 +140,7 @@ To use this, put something like this in your board header file::
>
>   Build:
>
> -After board configuration is done, fdt supported u-boot can be build in two
> +After the board configuration is done, fdt supported u-boot can be built in two
>   ways:
>
>   #  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE::
> @@ -177,12 +152,41 @@ ways:
>       $ make DEVICE_TREE=<dts-file-name>
>
>
> +Adding tweaks for U-Boot
> +------------------------
> +
> +It is strongly recommended that devicetree files in U-Boot be an exact copy of
> +those in Linux, so that it is easy to sync them up from time to time.
> +
> +U-Boot is of course a very different project from Linux, e.g. it operates under
> +much more restrictive memory and code-size constraints. Where Linux may use a
> +full clock driver with Common Clock Format (CCF) to find the input clock to the
> +UART, U-Boot typically wants to output a banner as early as possible before too
> +much code has run.
> +
> +A second difference is that U-Boot includes different phases. For SPL,
> +constraints are even more extreme and the devicetree is shrunk to remove
> +unwanted nodes, or even turned into C code to avoid access overhead.
> +
> +U-Boot automatically looks for and incluces a file with updates to the standard
> +devicetree for your board, searching for them in the same directory as the
> +main file, in this order::
> +
> +   <orig_filename>-u-boot.dtsi
> +   <CONFIG_SYS_SOC>-u-boot.dtsi
> +   <CONFIG_SYS_CPU>-u-boot.dtsi
> +   <CONFIG_SYS_VENDOR>-u-boot.dtsi
> +   u-boot.dtsi
> +
> +Only one of these is selected but of course you can #include another one within
> +that file, to create a hierarchy of shared files.
> +
>   Relocation, SPL and TPL
>   -----------------------
>
>   U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.
>
> -The full device tree is available to U-Boot proper, but normally only a subset
> +The full devicetree is available to U-Boot proper, but normally only a subset
>   (or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and
>   'SPL Support' in doc/driver-model/design.rst for more details.
>
> @@ -201,24 +205,24 @@ If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM
>   containing the board ID for example), it possible to start with a generic DTB
>   and then switch over to the right DTB after the detection. For this purpose,
>   the platform code must call fdtdec_resetup(). Based on the returned flag, the
> -platform may have to re-initiliaze the DM subusystem using dm_uninit() and
> +platform may have to re-initialise the DM subsystem using dm_uninit() and
>   dm_init_and_scan().
>
>
>   Limitations
>   -----------
>
> -U-Boot is designed to build with a single architecture type and CPU
> +Devicetree can help reduce the complexity of supporting variants of boards which
> +use the same SOC / CPU.
> +
> +However U-Boot is designed to build for a single architecture type and CPU
>   type. So for example it is not possible to build a single ARM binary
>   which runs on your AT91 and OMAP boards, relying on an fdt to configure
>   the various features. This is because you must select one of
>   the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
> -time. Similarly you cannot build for multiple cpu types or
> +time. Similarly U-Boot cannot be built for multiple cpu types or
>   architectures.
>
> -That said the complexity reduction by using fdt to support variants of
> -boards which use the same SOC / CPU can be substantial.
> -
>   It is important to understand that the fdt only selects options
>   available in the platform / drivers. It cannot add new drivers (yet). So
>   you must still have the CONFIG option to enable the driver. For example,
> @@ -227,4 +231,19 @@ but can use the fdt to specific the UART clock, peripheral address, etc.
>   In very broad terms, the CONFIG options in general control *what* driver
>   files are pulled in, and the fdt controls *how* those files work.
>
> -.. _dtspec: https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
> +History
> +-------
> +
> +U-Boot configuration was previous done using CONFIG options in the board
> +config file. This eventually got out of hand with nearly 10,000 options.
> +
> +U-Boot adopted devicetree around the same time as Linux and early boards
> +used it before Linux (e.g. snow). The two projects developed in parallel
> +and there are still some differences in the bindings with certain boards.
> +While there has beeen discussion of having a separate repo for devicetree
> +files, in practice the Linux kernel Git repository has become the place where
> +these are stored, with U-Boot taking copies and adding tweaks with u-boot.dtsi
> +files.
> +
> +.. _dtspec: https://www.devicetree.org/specifications/
> +.. _dtlist: https://www.spinics.net/lists/devicetree-compiler/
>

Re: [PATCH v2 3/3] doc: Add a note about why devicetree is used

[Heinrich Schuchardt]

On 8/2/21 2:57 AM, Simon Glass wrote:
> This question comes up every now and then with people coming from Linux.
> Add some notes about it so we can point to it in the mailing list.
>
> Signed-off-by: Simon Glass <sjg@chromium.org>

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

> ---
>
> Changes in v2:
> - Updates based on suggestions from Heinrich
>
>   doc/develop/devicetree/control.rst |  2 ++
>   doc/develop/devicetree/intro.rst   | 40 ++++++++++++++++++++++++++++++
>   2 files changed, 42 insertions(+)
>
> diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst
> index 9f2c21034d0..b33e2a7ead6 100644
> --- a/doc/develop/devicetree/control.rst
> +++ b/doc/develop/devicetree/control.rst
> @@ -152,6 +152,8 @@ ways:
>       $ make DEVICE_TREE=<dts-file-name>
>
>
> +.. _dttweaks:
> +
>   Adding tweaks for U-Boot
>   ------------------------
>
> diff --git a/doc/develop/devicetree/intro.rst b/doc/develop/devicetree/intro.rst
> index 344851327c7..36e8cc0d440 100644
> --- a/doc/develop/devicetree/intro.rst
> +++ b/doc/develop/devicetree/intro.rst
> @@ -2,3 +2,43 @@
>
>   Devicetree Introduction
>   =======================
> +
> +U-Boot uses a devicetree for configuration. This includes the devices used by
> +the board, the format of the image created with binman, which UART to use for
> +the console, public keys used for secure boot and many other things.
> +
> +See :doc:`control` for more information.
> +
> +Why does U-Boot put <thing> in the devicetree?
> +----------------------------------------------
> +
> +This question comes up a lot with people new to U-Boot, particular those coming
> +from Linux who are used to quite strict rules about what can go into the
> +devicetree.
> +
> +U-Boot uses the same devicetree as Linux but adds more things necessary for the
> +bootloader environment (see :ref:`dttweaks`).
> +
> +U-Boot does not have a user space to provide policy and configuration. It cannot
> +do what Linux does and run programs and look up filesystems to figure out how to
> +boot. So configuration and runtime information goes into the devicetree in
> +U-Boot.
> +
> +Of course it is possible to:
> +
> +- add tables into the rodata section of the U-Boot binary
> +- append some info to the end of U-Boot in a different format
> +- modify the linker script to bring in a file with some info in it
> +- put things in ACPI tables
> +- link in a UEFI hand-off block structure and put things in there
> +
> +but *please don't*. In general, devicetree is the sane place to hold U-Boot's
> +configuration.
> +
> +So, please, do NOT ask why U-Boot puts <thing> in the devicetree. It is the only
> +place it can go. It is a highly suitable data structure for just about anything
> +that U-Boot needs to know at runtime.
> +
> +Note, it is possible to use platdata directly so drivers avoid devicetreee in
> +SPL. But of-platdata is the modern way of avoiding devicetree overhead, so
> +please use that instead.
>

Re: [PATCH v2 3/3] doc: Add a note about why devicetree is used

[AKASHI Takahiro]

Heinrich,

On Sat, Aug 14, 2021 at 09:21:25AM +0200, Heinrich Schuchardt wrote:
> On 8/2/21 2:57 AM, Simon Glass wrote:
> > This question comes up every now and then with people coming from Linux.
> > Add some notes about it so we can point to it in the mailing list.
> > 
> > Signed-off-by: Simon Glass <sjg@chromium.org>
> 
> Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

If I correctly understand here, Simon's intent is to reassure
his opinion that a public key for capsule authentication should
be stored in a device tree, not be embedded in U-Boot binary.
So at the end, do you agree to reverting[1] Ilias' patch?

-Takahiro Akashi

[1] https://lists.denx.de/pipermail/u-boot/2021-August/456968.html


> > ---
> > 
> > Changes in v2:
> > - Updates based on suggestions from Heinrich
> > 
> >   doc/develop/devicetree/control.rst |  2 ++
> >   doc/develop/devicetree/intro.rst   | 40 ++++++++++++++++++++++++++++++
> >   2 files changed, 42 insertions(+)
> > 
> > diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst
> > index 9f2c21034d0..b33e2a7ead6 100644
> > --- a/doc/develop/devicetree/control.rst
> > +++ b/doc/develop/devicetree/control.rst
> > @@ -152,6 +152,8 @@ ways:
> >       $ make DEVICE_TREE=<dts-file-name>
> > 
> > 
> > +.. _dttweaks:
> > +
> >   Adding tweaks for U-Boot
> >   ------------------------
> > 
> > diff --git a/doc/develop/devicetree/intro.rst b/doc/develop/devicetree/intro.rst
> > index 344851327c7..36e8cc0d440 100644
> > --- a/doc/develop/devicetree/intro.rst
> > +++ b/doc/develop/devicetree/intro.rst
> > @@ -2,3 +2,43 @@
> > 
> >   Devicetree Introduction
> >   =======================
> > +
> > +U-Boot uses a devicetree for configuration. This includes the devices used by
> > +the board, the format of the image created with binman, which UART to use for
> > +the console, public keys used for secure boot and many other things.
> > +
> > +See :doc:`control` for more information.
> > +
> > +Why does U-Boot put <thing> in the devicetree?
> > +----------------------------------------------
> > +
> > +This question comes up a lot with people new to U-Boot, particular those coming
> > +from Linux who are used to quite strict rules about what can go into the
> > +devicetree.
> > +
> > +U-Boot uses the same devicetree as Linux but adds more things necessary for the
> > +bootloader environment (see :ref:`dttweaks`).
> > +
> > +U-Boot does not have a user space to provide policy and configuration. It cannot
> > +do what Linux does and run programs and look up filesystems to figure out how to
> > +boot. So configuration and runtime information goes into the devicetree in
> > +U-Boot.
> > +
> > +Of course it is possible to:
> > +
> > +- add tables into the rodata section of the U-Boot binary
> > +- append some info to the end of U-Boot in a different format
> > +- modify the linker script to bring in a file with some info in it
> > +- put things in ACPI tables
> > +- link in a UEFI hand-off block structure and put things in there
> > +
> > +but *please don't*. In general, devicetree is the sane place to hold U-Boot's
> > +configuration.
> > +
> > +So, please, do NOT ask why U-Boot puts <thing> in the devicetree. It is the only
> > +place it can go. It is a highly suitable data structure for just about anything
> > +that U-Boot needs to know at runtime.
> > +
> > +Note, it is possible to use platdata directly so drivers avoid devicetreee in
> > +SPL. But of-platdata is the modern way of avoiding devicetree overhead, so
> > +please use that instead.
> > 
>